These films are flexible and are constituted by a thin dielectric strip, made of polyester, for example, which is stored in roll form in a printer.
A latent electrostatic image is made up of ions deposited on one face of the intermediate film medium by selective electric discharge onto a zone of the film passing between a multitude of miniature print electrodes and adjacent counter-electrodes. The electrodes are usually distributed in one or more parallel rows and the counterelectrodes are usually disposed either opposite the electrodes and on the other side of the intermediate film medium, or else running alongside the electrodes and on the same side of the said intermediate film medium as the electrodes in a zone where electric discharge takes place.
The latent image is developed by inking it using liquid or powder ink particles which are electrically attracted to the electrostatic charges conveyed on the facing face of the intermediate film medium.
The inked face of the moving film is then locally pressed against one of the faces of a final recording medium in a transfer unit so as to transfer particles of ink to the final medium, with the ink particles being fixed on the final medium by the application of mechanical pressure which ensures close contact between the intermediate medium and the final medium.
The final recording medium is preferably a commonly-available and cheap medium of ordinary quality, for example a conventional non-coated paper. The image is formed at very high speed, typically about ten microseconds per point or per set of simultaneously formed points.
Since such printers must be capable of being installed in a wide variety of premises, and generally in non-specialized premises such as in offices where they may be simply stood on a table, it is important for such printers to be compact, silent, fast, and easy to operate by a wide variety of users who are not only untrained, but who may well also be clumsy.
In particular, there is a need for printers capable of printing on both sides of the final recording medium (i.e. of "duplex" or of "recto-verso" printing) in a single operation, at least so far the user is concerned.